Solid-state circuit breaker with self-diagnostic, self-maintenance, and self-protection capabilities

ABSTRACT

A solid-state circuit breaker (SSCB) with self-diagnostic, self-maintenance, and self-protection capabilities includes: a power semiconductor device; an air gap disconnect unit connected in series with the power semiconductor device; a sense and drive circuit that switches the power semiconductor device OFF upon detecting a short circuit or overload of unacceptably long duration; and a microcontroller unit (MCU) that triggers the air gap disconnect unit to form an air gap and galvanically isolate an attached load, after the sense and drive circuit switches the power semiconductor device OFF. The MCU is further configured to monitor the operability of the air gap disconnect unit, the power semiconductor device, and other critical components of the SSCB and, when applicable, take corrective actions to prevent the SSCB and the connected load from being damaged or destroyed and/or to protect persons and the environment from being exposed to hazardous electrical conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 17/010,122, filed Sep. 2, 2020, now U.S. Pat. No. 11,437,211,which claims the benefit of U.S. Provisional Patent Application No.62/895,182, filed Sep. 3, 2019.

BACKGROUND OF THE INVENTION

Circuit breakers are used in electrical distribution systems to protectelectrical circuits and their loads from short circuits and overloads.Conventional circuit breakers have an electromechanical construction,typically including an electromagnet that operates to separate thebreaker's contacts as quickly as possible when a short circuit occurs,and a thermally responsive bimetallic strip that separates the circuitbreaker's contacts after an overload has persisted in the circuit for anunacceptably long duration time.

Although conventional circuit breakers are effective at isolating faultsonce they have tripped, one significant drawback they have is that theyare slow—typically requiring several milliseconds to respond to andisolate a fault. To circumvent this limitation, efforts have been madein recent years to adapt high-power semiconductors i.e., “solid-state”devices) for circuit breaker applications. Solid-state devices areattractive since they can be controlled to isolate a fault in a matterof microseconds, compared to the several milliseconds it typically takesa conventional circuit breaker to isolate a similar fault. The fastreaction time is beneficial since it reduces the risk of fire, damage toelectrical equipment, and the possibility of arc flashes occurring.Further, because the operating characteristics of solid-stale devicesvary little from one part to another, circuit breakers can beconstructed from solid-state devices that exhibit precise andwell-controlled time-current characteristics. This is unlikeconventional circuit breakers, which, due to their thermal, magnetic andmechanical construction, exhibit wide variances in their time-currentcharacteristics.

Given their various advantages, solid-state circuit breakers have thepotential to supplant conventional circuit breakers in the not toodistant future. In order for that transition to occur, however,solid-state circuit breakers will need to be designed so that they aredurable and capable of operating for long periods of time unattended,without requiring an inordinate amount of human-involved oversight andmaintenance. Consistent with that goal, it would be desirable for thesolid-state circuit breaker to have the ability to monitor its owncritical functions, identify any deviations from its intended operation,diagnose and predict causes and sources of failure, protect itself fromdangerous conditions that might otherwise lead to its destruction and/orharm to persons and the environment, and lock itself down toelectrically and galvanically isolate its associated load whenconditions warrant.

BRIEF SUMMARY OF THE INVENTION

A solid-state circuit breaker (SSCB) with self-diagnostic,self-maintenance, and self-protection capabilities is disclosed. Anexemplary embodiment of the SSCB includes a power semiconductor device;an air gap disconnect unit connected in series with the powersemiconductor device; a sense and drive circuit that switches the powersemiconductor device OFF upon detecting a short circuit or overload ofunacceptably long duration; and a microcontroller unit (MCU) thattriggers the air gap disconnect unit to form an air gap and galvanicallyisolate an attached load, after the sense and drive circuit switches thepower semiconductor device OFF. The MCU is further configured to monitorthe operability of the air gap disconnect unit, the power semiconductordevice, and other critical components of the SSCB and, when applicable,take corrective actions to prevent the SSCB and the connected load frombeing damaged or destroyed and/or to protect persons and the environmentfrom being exposed to hazardous electrical conditions.

Further features and advantages of the invention, including a detaileddescription of the above-summarized and other exemplary embodiments ofthe invention, will now be described in detail with respect to theaccompanying drawings, in which like reference numbers are used toindicate identical or functionally similar elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram drawing of a solid-state circuit breaker(SSCB), according to one embodiment of the present invention;

FIG. 2 is a perspective view drawing of the SSCB depicted in FIG. 1 ,revealing some of its salient components, according to one embodiment ofthe present invention;

FIG. 3 is an exploded view drawing of the SSCB depicted in FIG. 1 ,further revealing some of its salient components, according to oneembodiment of the present invention;

FIG. 4 is a perspective view drawing of the air gap disconnect unit ofthe SSCB depicted in FIG. 1 , according to one embodiment of the thepresent invention;

FIG. 5 is a front perspective view drawing of the SSCB depicted in FIG.1 , illustrating how it is preferably housed within a housing andrevealing the SSCB's electronic display, ON/OFF/STANDBY buttons, and airgap disconnect unit RESET and RELEASE buttons, according to oneembodiment of the present invention;

FIG. 6 is a line-side perspective view drawing of the SSCB depicted inFIG. 1 , revealing the SSCB's Line-IN terminals, according to oneembodiment of the present invention;

FIG. 7 is a load-side perspective view drawing of the SSCB depicted inFIG. 1 , revealing the SSCB's Line-OUT terminals, according to oneembodiment of the present invention;

FIG. 8 is a flowchart illustrating a method the MCU of the SSCB performsto monitor the core functionality of the SSCB, and how the MCU respondswhen it determines that the SSCB's core functionality has failed orlikely failed, according to one embodiment of the present invention;

FIG. 9A is a portion of a flowchart illustrating a method the MCU of theSSCB performs to monitor and diagnose whether the air gap disconnectunit in the SSCB has failed closed, or has likely failed closed, and theactions the SSCB takes upon determining that the air gap disconnect unithas failed closed, or likely failed closed, according to one embodimentof the present invention;

FIG. 9B is a portion of a flowchart illustrating a method the MCU of theSSCB performs to monitor and diagnose whether the air gap disconnectunit in the SSCB has failed closed, or has likely failed closed, and theactions the SSCB takes upon determining that the air gap disconnect unithas failed closed, or likely failed closed, according to one embodimentof the present invention;

FIG. 10A is a portion of a flowchart illustrating a method the MCU ofthe SSCB performs to monitor and diagnose whether the air gap disconnectunit in the SSCB has failed open, or has likely failed open, and theactions the SSCB takes upon determining that the air gap disconnect unithas failed open, or likely failed open, according to one embodiment ofthe present invention;

FIG. 10B is a portion of a flowchart illustrating a method the MCU ofthe SSCB performs to monitor and diagnose whether the air gap disconnectunit in the SSCB has failed open, or has likely failed open, and theactions the SSCB takes upon determining that the air gap disconnect unithas failed open, or likely failed open, according to one embodiment ofthe present invention;

FIG. 11 is a flowchart illustrating a method the MCU of the SSCBperforms to monitor and diagnose the vitality, efficacy, and operabilityof an air gap disconnect capacitor in the SSCB, and the actions the SSCBtakes upon determining that the air gap disconnect capacitor has failedor is failing;

FIG. 12 is a block diagram illustrating how, in one embodiment of theSSCB, the SSCB's MCU is configured to generate a periodic heartbeatsignal that an external watchdog circuit continuously monitors toconfirm the vitality and operability of the MCU;

FIG. 13 is a flowchart illustrating a method the MCU of the SSCBperforms to monitor the vitality, efficacy, and operability of theSSCB's AC/DC converter, which serves as the primary DC power supply forthe MCU and other DC-powered electronics within the SSCB, and theactions the SSCB takes upon determining that the AC/DC converter hasfailed, or is likely failing, in accordance with one embodiment of thepresent invention;

FIG. 14 is a flowchart illustrating a method the MCU of the SSCBperforms to monitor the vitality, efficacy, and operability of the powerfield-effect transistors (FETs) of a FET power module in the SSCB, andthe actions the MCU takes upon determining that one or more of the powerFETs has/have failed, has/have likely failed, or is/are failing, inaccordance with one embodiment of the present invention;

FIG. 15 is flowchart illustrating a method the MCU of the SSCB performsto monitor the vitality, efficacy, and operability of the power FETs ofthe SSCB's FET power module, and the actions the MCU takes upondetermining that one or more of the power FETs has/have failed, has/havelikely failed, or is/are failing, in accordance with one embodiment ofthe present invention;

FIG. 16 is a flowchart illustrating a method the MCU of the SSCBperforms in response to sensed/measured power FET junction temperaturessensed/measured by thermistors mounted in the SSCB's FET power module,and what actions the MCU takes upon determining that the junctiontemperature T_(J) of one or more of the FETs in the FET power moduleexceeds a predetermined maximum permissible junction temperatureT_(MAX), in accordance with one embodiment of the present invention;

FIG. 17 is a flowchart illustrating a method the MCU of the SSCBperforms to detect the failure or likely failure of one or more of thepower FETs in the FET power module, and the actions the MCU takes upondetermining that one or more of the power FETs has/have failed,according to one embodiment of the present invention;

FIG. 18 is a flowchart illustrating a method the MCU of a single-phaseversion of the SSCB performs to detect the failure or likely failure ofthe SSCB's power FET, and the actions the MCU takes upon determiningthat the power FET has failed, has likely failed, or is likely failing,according to one embodiment of the present invention;

FIG. 19 is a flowchart illustrating a method the MCU of the SSCBperforms to monitor the performance and diagnose any failure of line-OUTHall effect sensors in the SSCB, and what actions the MCU takes upondetermining that one or more of the Hall effect sensors has failed, hasfailed, or is likely failing, according to one embodiment of the presentinvention;

FIG. 20 is a flowchart illustrating a method the MCU of the SSCBperforms to verify that the SSCB's air gap disconnect unit hassuccessfully reengaged in response to a person pressing the SSCB's RESETbutton, and what actions the MCU takes upon determining that the air gapdisconnect unit has failed to successfully reengage, according to oneembodiment of the present invention;

FIG. 21 is a schematic drawing of the salient components of the FETpower module of the SSCB, according to one embodiment of the presentinvention, highlighting the connectivity of the power FETs and placementof various surge protection devices (SPDs) in the FET power module, inaccordance with one embodiment of the present invention;

FIG. 22 is a flowchart illustrating a method the MCU of the SSCBperforms to monitor the health of the SPDs in the FET power module, andwhat actions the MCU takes upon determining that one or more of the SPDshas/have failed, according to one embodiment of the present invention;

FIG. 23 is a flowchart illustrating a method the MCU of the SSCBperforms to monitor the operational status of one or more microswitchesthat are configured to indicate a physical displacement of the SSCB'sRELEASE button, and what actions the MCU performs upon determining thatone or more of the one or more microswitches has failed, according toone embodiment of the present invention;

FIG. 24 is a drawing showing a plurality of the SSCBs configured in a anelectrical distribution panel, how the SSCBs are in electricalcommunication with a communications and control (comm/control) bus witha head-end interface, and how an external local or remotely locatedcomputer (a tablet computer in the drawing), can be used performon-demand SSCB diagnostics and maintenance, and schedule, via auser-interactive graphical user interface (GUI), periodic SSCBdiagnostic and/or maintenance, in accordance with one embodiment of thepresent invention;

FIG. 25 is a block diagram highlighting the salient components of theexternal local or remotely controlled computer used to perform theon-demand SSCB diagnostics and maintenance, and schedule, via theuser-interactive GUI, periodic SSCB diagnostic and/or maintenance;

FIG. 26A is a side view drawing of a modified air gap disconnect unitwhen configured in an engaged state, according to one embodiment of thepresent invention;

FIG. 26B is a top plan view drawing of the modified air gap disconnectunit when configured in the engaged state;

FIG. 27A is a side view drawing of the modified air gap disconnect unitwhen configured in a disengaged state;

FIG. 27B is a top plan view drawing of the modified air gap disconnectunit when configured in the disengaged state;

FIG. 28A is a side view drawing of the modified air gap disconnect unitwhen configured in a mechanically locked-out state;

FIG. 28B is a top plan view drawing of the modified air gap disconnectunit when configured in the mechanically locked-out state;

FIG. 29A is a portion of a flowchart illustrating a method the SSCBdepicted in FIG. 1 performs when modified to include the modified airgap disconnect unit depicted in FIGS. 26A, 26B, 27A, 27B, 28A and 28B,according to one embodiment of the present invention; and

FIG. 29B is a portion of a flowchart illustrating a method the SSCBdepicted in FIG. 1 performs when modified to include the modified airgap disconnect unit depicted in FIGS. 26A, 26B, 27A, 27B, 28A and 28B,according to one embodiment of the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1 , there is shown a block diagram of a solid-statecircuit breaker (SSCB) 100 with self-diagnostic, self-maintenance, andself-protection capabilities, according to one embodiment of the presentinvention. The principal components of the SSCB 100 include: amicrocontroller unit (MCU) 102; a sense and drive circuit 104, afield-effect transistor (FET) power module 106; and an air gapdisconnect unit 108. The air gap disconnect unit 108 has three air-gapcontact switches 114 and the FET power module 106 has three power FETs116, each one of which is configured so that its drain-source path is inseries with one of the air gap contact switches 114. In other words, theFET power module 106 and air gap disconnect unit 108 are connected inseries, between the Line-IN terminals and Line-OUT terminals 110 and112. During normal operating conditions, the three air-gap contactswitches 114 in the air gap disconnect unit 108 are closed and the threepower FETs 116 in the FET power module 106 are ON. This allows linecurrents entering the Line-IN terminals 110 from a power source (e.g.,alternating currents (AC) distributed to the SSCB 100 Line-IN terminals110 from within a distribution panel) to flow to a load (not shown)connected to the Line-OUT terminals 112 (e.g., via electrical cablesrouted to the load in a commercial building or residence). However, uponthe SSCB 100 detecting a short circuit or overload of unacceptably longduration, the sense and drive circuit 104 switches the power FETs 116OFF to prevent any further current from flowing to the load. Meanwhile,or shortly after the power FETs 116 are switched OFF, the MCU 102generates a solenoid trigger signal, which the air gap disconnect unit108 responds to, to ‘disengage,’ open the air gap contact switches 114and galvanically isolate the load. Note that the exemplary embodiment ofthe SSCB 100 depicted in FIG. 1 and in other drawings in this disclosureis a three-phase device. Accordingly, it has three Line-IN terminals110, three air-gap contact switches 114, three power FETs 116, and threeLine-OUT terminals 112. A single-phase version of the SSCB would havejust one of each of these components, as will be appreciated andunderstood by those of ordinary skill in the art. It should also bementioned that in a preferred embodiment of the invention the power FETs116 comprise silicon carbide (SiC) MOSFETs. However, other types ofpower semiconductor devices may be alternatively used (e.g., GaN highelectron mobility transistors (HEMTs)), as will be appreciated andunderstood by those of ordinary skill in the art.

When the SSCB 100 is in the ‘tripped’ state, the power FETs 116 arefully capable by themselves of isolating the load from the input powersource. However, because governmental, regulatory, and certificationbodies usually require the load to be galvanically isolated from theinput power source when the circuit breaker is in the tripped state, theSSCB 100 is preferably (though not necessarily in every embodiment ofthe invention) equipped with the air gap disconnect unit 108, which,when ‘disengaged,’ forms an air gap between the Line-IN and Line-OUTterminals 110 and 112. Among other components, the air gap disconnectunit 108 includes a large (i.e., high capacitance) air gap disconnectcapacitor 117 (for example, 10,000 microfarads or higher), a solenoid118, and a switch 120 that selectively couples the terminals of the airgap disconnect capacitor 117 across the coil of the solenoid 118. Thephysical relationship of the air gap disconnect capacitor 117 and thesolenoid 118 in one exemplary embodiment of the SSCB 100 can be seen inthe perspective and exploded view drawings of the SSCB 100 presented inFIGS. 2 and 3 , as well as the perspective view of the air gapdisconnect unit 108 by itself in FIG. 4 .

In one embodiment of the invention, the air gap disconnect unit 108 isdesigned so that it can be triggered to disengage and form an air gapbetween the Line-IN and Line-OUT terminals 110 and 112 automatically (byactuating the solenoid 118), manually (without the assistance of thesolenoid 118 but in response to a person pressing a RELEASE button 122),or in response to a command from a local or remote computer that isconfigured to interact with the SSCB 100 via a communications andcontrol (comm/control) bus 124. Automatic triggering occurs upon thesense and drive circuit 104 detecting a short circuit or otherelectrical anomaly (e.g., overload of unacceptably long duration) in theSSCB's 100's load circuit. When that occurs, the sense and drive circuit104 generates and applies gating disable signals to the gates of thepower FETs 116 in the FET power module 106, causing the power FETs 116to switch OFF in a matter of microseconds. The power FETs 116 can alsobe instructed to delay their switch OFF to allow for switch OFF during azero cross of the current. The ability to switch OFF during a zero pointof current reduces the disturbance in the upstream power system.Meanwhile or shortly after the power FETs 116 have been switched OFF,the MCU 102 generates a solenoid trigger signal, which closes switch 120(see FIG. 1 ) so that the terminals of the air gap disconnect capacitor117 are connected across the coil of the solenoid 118. The SSCB 100 isdesigned so that the air gap disconnect capacitor 117 remains chargedduring normal operating conditions. In one embodiment of the invention,an AC/DC converter 126, which serves as the main DC power supply for theDC powered electronics in the SSCB 100 (e.g., the MCU 102, CRM 103, andDC components on the sense and drive circuit board 104), is furtherconfigured to charge the air gap disconnect capacitor 117 and maintainit in a fully charged state. Accordingly, when the solenoid triggersignal is applied to switch 120 to cause it to close and the terminalsof the air gap disconnect capacitor 117 are then connected across thecoil of the solenoid 118, a large current begins to flow through thesolenoid's coil. The large coil current creates, in turn, a magneticfield that causes the solenoid 118 to pull its plunger 128 inside thesolenoid's housing. As the plunger 128 is pulled into the solenoidhousing it engages a latch 130, which rotates about a pivot and swingsaway from a top lip 132 of a holster 136. (See FIGS. 2-4 .) Under normaloperating conditions, the latch 130 is positioned so that it holds theholster 136 down (by pressing down on the top lip 132), thereby keepingthe air gap contact switches 114 closed. However, when the solenoid 118is triggered and the solenoid's 118's plunger 128 causes the latch 130to rotate and swing away from the top lip 132, the latch 130 no longerholds the holster 136 down and normally-compressed disconnect springs138 decompress to force holster 136 upwards. Forcing the holster 136 upcauses the air gap contact switches 114 to open, resulting in formationof an air gap between the Line-IN and Line-OUT terminals 110 and 112.This air gap galvanically isolates the load from the input power sourceapplied to the Line-IN terminals 110, so with the power FETs 116 OFF andthe air gap disconnect unit 108 disengaged with an air gap formedbetween the Line-IN and Line-OUT terminals 110 and 112, the SSCB 100 isin a fully ‘tripped’ state.

In one embodiment of the invention the air gap disconnect unit 108 isfurther designed so that it can be disengaged manually, specifically, inresponse to a person pressing RELEASE button 122. When a person pressesthe RELEASE button 122, linking member 140 (see FIGS. 2-4 ) pushesagainst a cammed surface 142 on the latch 130, causing the latch 130 torotate and release (disengage) from the top lip 132 of the holster 136and allowing normally-compressed disconnect spring 138 to decompress andforce holster 136 up to open air gap contact switches 114 and form theair gap. As the holster 136 is forced up, it also pushes against andlifts a RESET button 144 to expose lockout-tagout (LOTO) hole 146,through which a service or maintenance (service/maintenance) worker caninsert a padlock or other locking device to complete LOTO safetyprocedure. Note that RESET button 144 will pop up to reveal LOTO hole146 whether the air gap disconnect unit 108 is triggered to disengageautomatically (by the MCU 102 actuating the solenoid 118) or manually(by a person pressing RELEASE button 122). Completing the LOTO safetyprocedure ensures that the service/maintenance worker or other personwill not inadvertently or accidentally reengage the air gap disconnectunit 108. After service/maintenance has been completed, theservice/maintenance worker can then remove the padlock or other lockingdevice and reengage the air gap disconnect unit 108 by pushing RESETbutton 144. As the RESET button 144 is pressed it engages the holster136 and recompresses the disconnect springs 138. It also engages therotating latch 130, so that it reengages the top lip 132 of the holster136, to hold the holster 136 down once again and close the air gapcontact switches 114. Further details of an SSCB 100 that includes asimilar air gap disconnect unit can be found in co-pending and commonlyassigned U.S. patent application Ser. No. 16/898,538, entitled“Solid-State Circuit Breaker with Galvanic Isolation,” now U.S. Pat. No.11,322,916, which is incorporated herein by reference.

It should be mentioned that the SSCB 100 preferably includes anadditional lockout mechanism 160 that can be activated by the SSCB 100itself (i.e., without any human participation) to prevent the RESETbutton 144 from being pressed and the air gap disconnect unit 108 frombeing re-engaged when conditions warrant. This additional lockoutmechanism 160 can be activated by the SSCB 100 regardless of whether apadlock or other locking device is inserted through the LOTO hole 146 ofthe RESET button 144. In one embodiment of the invention the additionallockout mechanism 160 includes a secondary solenoid smaller in sizecompared to the primary solenoid 118 used to disengage the air gapdisconnect unit 108. After the MCU 102 has determined that one of theSSCB's 100's principal components or critical functions has failed or islikely failing and the MCU 102 has triggered the primary solenoid 118 todisengage the air gap disconnect unit 108 (for example, after performingone of the various self-diagnostic, self-maintenance, andself-protection methods described herein), it triggers the secondarysolenoid. The plunger of the secondary solenoid is configured so that itengages the RESET button 144 (or engages some other mechanical linkageconnected to the RESET button 144) and prevents the RESET button 144from being pressed into the SSCB's 100's enclosure. The secondarylockout mechanism 160 thus prevents a person from attempting tore-engage the air gap disconnect unit 108 after the SSCB 100 hasdetermined that hazardous conditions may be present and/or hasdetermined that one of its principal components or critical functionshas/have failed or is likely failing.

In the exemplary embodiment of the SSCB 100 depicted in FIGS. 1 and 2 ,the MCU 102 is mounted on a control board 105, along with computerreadable memory (CRM) 103. The CRM 103 comprises flash memory and/orelectrically erasable read only memory (EPROM) for storing firmware andsoftware that directs the MCU's 102's operation, as well as randomaccess memory (RAM) which the MCU 102 utilizes in executing the firmwareand software instructions. As will be appreciated by those of ordinaryskill in the art, the CRM 103 may be external to the MCU 102 (asdepicted in FIG. 1 ), embedded in the MCU 102, or may comprise computerreadable memory that is partly embedded in the MCU 102 and partlyexternal to the MCU.

Preferably the SSCB 100 is enclosed within a housing (i.e., an‘enclosure’), as illustrated in FIGS. 5-7 , and includes ON, OFF, andSTANDBY buttons (ON/OFF/STANDBY) 107, 109 and 111 that a person canpress and an electronic display 113. In one embodiment of the SSCB 100the electronic display 113 is an electronic ink display, which is adisplay technology that allows information currently being displayed tocontinue to be displayed even if electrical power to the display isdisrupted or completely removed. (Note that ribbon cable 119 (see FIG. 2) provides the electrical connectivity between the control board 105 andthe ON/OFF/STANDBY buttons 107, 109 and 111.) In one embodiment of theinvention the ON/OFF/STANDBY buttons 107, 109 and 111 are translucentand colored green, red, and yellow, respectively, and light-emittingdiodes (LEDs) (not shown in the drawings) are configured beneath eachbutton. Whichever LED is illuminated indicates and emphasizes thepresent state of the SSCB 100. Specifically, when the ON button 107 (thegreen colored button with the “I” indicator) is pressed ON and itsunderlying LED is illuminated, the green emphasis indicates that theSSCB 100 is in a fully operational state (air gap disconnect unit 108engaged (air gap contact switches 114 closed) and power FETs 116 in FETpower module 106 ON). When the STANDBY button 109 (yellow button markedwith an “X”) is pressed and its underlying LED is illuminated, theyellow emphasis indicates that the SSCB 100 is in a standby state (airgap disconnect unit 108 engaged but power FETs 116 in FET power module106 OFF). (Note that a transition from the STANDBY state to the ONstate, can then be made, if desired, by simply pushing the ON (green)button 107.) Finally, the OFF red button 111, which is not actually apressable button in the exemplary embodiment of the SSCB 100 describedhere (but could be in an alternative design) is illuminated by itsunderlying LED when both the air gap disconnect unit 108 is disengaged(air gap contact switches 114 open) and the power FETs 106 in FET powermodule 116 are OFF. The red emphasis indicates that the SSCB 100 is in a‘tripped,’ i.e., fully OFF state (power FETs 116 in FET power module 106OFF and air gap disconnect unit 108 disengaged). The reason that the OFFred button 111 does not need to be a pressable button is that thatfunctionality is already provided by RELEASE button 122, which whenpressed causes the SSCB's electronics and driver circuitry to switch thepower FETS 116 in FET power module 106 OFF and soon thereafter triggerthe air gap disconnect unit 108 to disengage.

FIG. 7 , which is a load-side perspective view of the enclosed SSCB 100,reveals the Line-OUT terminals 112, which in this particular exemplaryembodiment include load-side connection lugs 148, to which power cablescan be connected and routed to the load. It also reveals acommunications and control (comm/control) bus shield 150 which protectsan electrical comm/control bus connector that connects the SSCB 100 tothe comm/control bus 124 when the SSCB 100 is installed in a panelboard,similar to as described in co-pending and commonly assigned U.S. patentapplication Ser. No. 16/898,569, entitled “Distribution Panel forIntelligently Controlled Solid-State Circuit Breakers,” now U.S. Pat.No. 11,309,690, which is incorporated herein by reference. Thecomm/control bus 124, which may be an inter-IC (I2C) or controller areanetwork (CAN) bus, for example, provides the SSCB 100 the ability tocommunicate with, and to be controlled by, a local or remote computer,similar to as described in commonly assigned U.S. Pat. No. 10,541,530,entitled “Hybrid Air-Gap/Solid-State Circuit Breaker,” and commonlyassigned U.S. Pat. No. 10,276,321, entitled “Dynamic Coordination ofProtection Devices in Electrical Distribution Systems,” both which arealso incorporated herein by reference. It also provides the SSCB 100 theability to report its operational status (in real time or non-real-time)to the local or remote computer, for example, after performing a set ofscheduled or on-demand diagnostic tests, as will be explained in furtherdetail below.

The control board 105 serves as the ‘brains’ of the SSCB 100 and in oneexemplary embodiment of the SSCB 100 includes the MCU 102 and CRM 103.The firmware and other computer program instructions (i.e., software)stored in the CRM 103 and retrieved and executed by the MCU 102 controlthe general operation of the SSCB 100, including instructions thatdetermine and control if, how, and when the MCU 102 interacts with othercomponents in the SSCB 100, and instructions and protocol that allow theMCU 102 to communicate with, and to be controlled by, other devices(e.g., an external computer) over the comm/control bus 124. In someembodiments of the SSCB 100 the computer program instructions furtherinclude instructions which when executed by the MCU 102 give the MCU 102the ability to: 1) monitor and diagnose the vitality, efficacy, andoperability of the air gap disconnect unit 108, including determining,when appropriate or necessary, whether the air gap disconnect unit 108has failed open or failed closed; 2) monitor and diagnose the vitality,efficacy, and operability of the power FETs 116 in the FET power module106 and direct the sense and drive circuit 104 to switch the power FETs116 OFF when conditions warrant; 3) generate the solenoid trigger signalto cause the air gap disconnect unit 108 to disengage and form an airgap between the Line-IN and Line-OUT terminals 110 and 112 whenconditions warrant; 4) monitor the vitality, efficacy, and operabilityof the AC/DC converter 126 and direct the sense and drive circuit 104 toswitch the power FETs 116 OFF if the AC/DC converter 126 has failed oris determined to be failing (using, if necessary, energy stored in theair gap disconnect capacitor 117); 5) monitor and diagnose the vitality,efficacy, and operability of the air gap disconnect capacitor 117 andgenerate a capacitor bypass signal that allows the AC/DC converter 126to bypass the air gap disconnect capacitor 117 if the MCU 102 determinesthat the air gap disconnect capacitor 117 has failed or is failing; 6)generate a watchdog signal that an external watchdog utilizes to monitorthe vitality, efficacy, and operability of the MCU 102; 7) monitor anddiagnose failures and likely or probable failures of the current andvoltage sensors 154 and 156 used to sense the line currents and voltagesat the inputs and outputs of the FET power module 106; 8) monitor thevitality, efficacy, and operability of surge protection devices (SPDs)that serve to prevent the power FETs 116 from being exposed to highvoltages, and locking the SSCB 100 down if one or more of the SPDshas/have failed or has/have degraded beyond their practical utility; 9)direct and control the SSCB's 100's electronic display 113 to displayinformation (both real-time and non-real-time) concerning the vitality,efficacy, and operability of the SSCB 100 and its various components;10) report information (both real-time and non-real-time) concerning thevitality, efficacy, and operability of the SSCB 100, via thecomm/control bus 124, to the system overseer (e.g., electrician,residence/building owner, or electrical utility); and 11) recommendcorrective actions the system overseer might possibly take to address orremedy any particular failure or problem the SSCB 100 may haveexperienced or is currently experiencing. A recommended correctiveaction could include, for example, turning OFF an upstream circuitbreaker (which might also be an SSCB similar in construction to theexemplary SSCB 100 described herein) based on a fault diagnosed in adownstream SSCB. Taking such corrective action would reduce the risk todownstream equipment should the fault compromise or eliminate thedownstream SSCB's protective functions. These and other functions andcapabilities of the SSCB 100 are discussed in further detail below.

FIG. 8 is a flowchart illustrating a method 800 the MCU 102 performs(together with other cooperating components of the SSCB 100) to verifythe basic (i.e., “core”) functionality of the SSCB 100, and what actionsthe MCU 102 takes upon determining that the core functionality of theSSCB 100, specifically, tripping when required, has failed or likelyfailed. According to this method, the SSCB 100 is assumed to beconfigured in an electrical distribution system with an input powersource (e.g., the AC mains) attached to the SSCB's line-IN terminals 110and a load attached to the line-OUT terminals 112. First, at step 802the SSCB 100 is switched ON (power FETs closed and air gap disconnectunit engaged (no air gap). At step 804 the MCU 102 reduces the tripthreshold of the SSCB 100 to and past the nominal current of the load.If the SSCB's core functionality is operating properly, the SSCB 100should trip (power FETs 116 open, air gap disconnect unit disengages) asthe trip threshold is adjusted below the nominal load current. Todetermine whether the SSCB 100 did trip, as intended, step 806 isperformed. Specifically, at step 806 the MCU 102 determines the voltagedifference ΔV between the line-IN and line-OUT terminals 110 and 112.If, at decision 808, ΔV is determined to be sufficiently greater thanzero to indicate galvanic isolation of the load (“YES” at decision 808),the MCU 102 is able to properly conclude that the SSCB's corefunctionality is operating correctly, the trip threshold is set back tonormal at step 810, and at step 812 the power FETs 116 are quicklyswitched back ON (so current to the load is interrupted only brieflyduring the test), the air gap disconnect unit 108 is reengaged to closethe air gap, and the method 800 ends. However, if at decision 808 theMCU 102 determines that ΔV between the line-IN and line-OUT terminals110 and 112 is zero or not sufficiently greater than zero to indicateproper galvanic isolation (“NO” at decision 808), the MCU 102 concludesthat the SSCB's core functionality has failed and at step 814 alerts thesystem overseer that SSCB's core functionality has failed. Failure ofthe SSCB's 100's core functionality could be attributable to, forexample, a damaged power FET 116 in the FET power module 106, amalfunctioning air gap disconnect unit 108, a damaged or destroyed senseand drive circuit 104, a damaged current and/or current and voltagesensor 154 and 156, or could have been caused by some other damaged ormalfunctioning component. In some embodiments of the invention the MCU102 reports the failure to a remote computer, via the comm/control bus124, and/or directs the SSCB to display the failure information on theSSCB's electronic display 113. To further isolate and identify thesource of the core functionality failure, some or more of thediagnostics tests discussed below can be performed following a “NO” atdecision 808.

In the core functionality diagnostic method 800 just described the airgap disconnect unit 108 must be re-engaged (e.g., by a user pressing theRESET button 144) after the core functionality of the SSCB 100 has beenverified (i.e., following a “YES” at decision 808). In an alternativemethod, the MCU 102 does not trigger the air gap disconnect unit 108 todisengage during the core functionality diagnostic check, but insteadmaintains the air gap disconnect unit 108 in an engaged positionthroughout the test (no air gap between the line-IN and line-OUTterminals 110 and 112). According to this alternative approach, if atdecision 808 the MCU 102 determines that ΔV is zero or not sufficientlygreater than zero (“NO” at decision 808), the MCU 102 is able toproperly conclude that either one or more of the power FETs 116 has/havefailed and/or that one or more of the current/voltage sensors 154 and156 has/have failed or has/have likely failed, and at step 814 thesystem overseer is then alerted of the failure or likely failure. Onebenefit of this alternative core functionality diagnostic check is thatthere is no need for a person (i.e., “user”) to re-engage the air gapdisconnect unit 108 following the diagnostic check (since the MCU 102does not trigger the air gap disconnect unit 108 to disengage during thecheck). Another benefit is that the speed of the diagnostic check ismuch faster compared to the approach in which the air gap disconnectunit 108 is disengaged and then re-engaged, thereby resulting in almostimperceptible effect on current flow to the load. It should also bepointed out that this alternative core functionality diagnostic checkcould also be beneficially used in embodiments of the SSCB 100 that donot include an air gap disconnect unit 108, in other words, inembodiments of the SSCB 100 that only include power semiconductorsbetween the line-IN and line-OUT terminals 110 and 112 and thatconsequently rely solely on the power semiconductors to isolate the loadin the event of a fault or overload.

FIG. 9 (FIGS. 9A and 9B inclusive) is a flowchart of a method 900 theMCU 102 performs (together with other cooperating components of the SSCB100) to monitor and diagnose the vitality, efficacy, and operability ofthe air gap disconnect unit 108, specifically, to monitor and diagnosewhether the air gap disconnect unit 108 has failed closed (perhaps dueto one or more of the air gap contact switches 114 failing closedwithout any ability to reopen) and how the SSCB 100 responds after theMCU 102 determines that the air gap disconnect unit 108 has, in fact,failed closed. According to this method 900, the MCU 102 is alsoprovided the ability to detect the possible failure of the Line-INand/or Line-OUT current/voltage sensors 154 and 156. At step 902 the MCU102 commands the air gap disconnect unit 108 to disengage and form anair gap (by generating a solenoid trigger signal to close switch 120,connect the air gap disconnect capacitor 117 across the coil of thesolenoid 118, and open the air gap contact switches 114, as previouslydescribed) and after the air gap has formed commands the sense and drivecircuit 104 to switch the power FETs 116 ON. At steps 904 and 906, theinput and output voltages for all three phases are measured at both theLine-IN and Line-OUT sides of the FET power module 106. Thesensed/measured Line-IN and Line-OUT voltages are directed to the MCU102, and in step 908 the MCU 102 compares the Line-IN and Line-OUTvoltages V(Line-IN) and V(Line-OUT) or computes the difference betweenthem for each phase. Then, at decision 910, if the MCU 102 determinesthat for any given phase V(Line-IN)<V(Line-OUT) (“YES” at decision 910),the MCU 102 is able to properly conclude that either the air gapdisconnect unit 108 has failed closed and/or that one or more of theLine-IN and Line-OUT current/voltage sensors 154 and 156 has/havefailed. To eliminate the possibility that one or more of the Line-OUTcurrent/voltage sensors 156 has/have failed, step 912 and decision 914are performed. Specifically, at step 912 the MCU 102 directs the senseand drive circuit 104 to switch OFF the power FET 116 that is in thephase in which V(Line-IN)<V(Line-OUT) was determined. Then, the Line-OUTcurrent/voltage sensor 156 measures V(Line-OUT) at the output of thepower FET 116 that has just been switched OFF, and at decision 914 theMCU 102 determines whether V(Line-OUT) goes low. If V(Line-OUT) does golow, the MCU 102 is able to conclude that the air gap disconnect unit108 has failed closed and that one more of the Line-IN current/voltagesensors 154 has/have likely failed. Since both are serious problems, atstep 916 the MCU 102 directs the sense and drive circuit 104 to switchthe remaining power FETs 116 OFF and prevents any further attempt toswitch the power FETs 116 back ON, and at step 918 reports to the systemoverseer that the air gap disconnect unit 108 has failed closed and thatone or more of the Line-IN current/voltage sensors 154 has also likelyfailed. The MCU 102 may also report the failure or likely failure to aremote computer, via the comm/control bus 124, and/or direct the SSCB's100's display to display the failure information on its electronicdisplay 113, as indicated by step 918.

If V(Line-OUT) does not go low at decision 814 (“NO” at decision 914),the MCU 102 is able to conclude that the air gap disconnect unit 108 hasnot failed closed but that one or more of the Line-OUT current/voltagesensors 156 has/have likely failed. Accordingly, at step 922 the MCU 102directs the sense and drive circuit 104 to switch the remaining powerFETs 116 OFF, prohibits the air gap disconnect unit 108 fromre-engaging, and prevents any further attempt to switch the power FETs116 back ON. In this state the SSCB 100 is locked down until it can beserviced/repaired by a qualified electrician or engineer. Finally, atstep 924 the MCU 102 reports the probable failure of one or more of theLine-OUT current/voltage sensors 156 to the system overseer and/ordirects the SSCB 100 to indicate on its electronic display 113 that oneor more of the Line-OUT current/voltage sensors 156 has/have likelyfailed.

At decision 910 if the MCU 102 determines that V(Line-IN) is not lessthan V(Line-OUT) in any of the three phases (“NO” at decision 810), theMCU 102 is able to properly conclude that the air gap disconnect unit108 has not failed closed and the Line-IN current/voltage sensors 154are operating as intended. However, that determination does not byitself foreclose the possibility that one or more of the Line-OUTcurrent/voltage sensors 156 has/have failed. To ensure that the Line-OUTcurrent/voltage sensors 156 are working as intended, decision 920 isperformed, specifically, querying whether for any given phaseV(Line-IN)>V(Line-OUT). If for any given phase V(Line-IN)>V(Line-OUT)(“YES” at decision 920), the MCU 102 is able to properly conclude thatalthough the air gap disconnect unit 108 has not failed closed and theLine-IN current/voltage sensors 154 are operating as intended, it islikely that one or more of the Line-OUT current/voltage sensors 156has/have failed. Accordingly, step 922 is performed to lock down theSSCB 100, and, finally, at step 924 the MCU 102 reports the probablefailure of one or more of the Line-OUT current/voltage sensors 156 tothe system overseer and/or directs the SSCB 100 to indicate on itselectronic display 113 that one or more of the Line-OUT current/voltagesensors 156 has/have likely failed.

In addition to being programmed to monitor and diagnose whether the airgap disconnect unit 108 has failed closed, in one embodiment of theinvention the MCU 102 is programmed to also monitor and diagnose whetherthe air gap disconnect unit 108 has failed open. FIG. 10 (FIGS. 10A and10B inclusive) is a flowchart of a method 1000 the MCU 102 performs inthis regard (together with other cooperating components of the SSCB100). Prior to the start of the method 1000, it is assumed that all ofthe power FETs 116 are ON and the air gap disconnect unit 108 is engaged(air gap contact switches 114 closed). Then, at steps 1002 and 1004 theinput and output voltages for all three phases are measured at both theLine-IN and Line-OUT sides of the FET power module 106. At decision1006, using the current/voltage measurements it receives from theLine-IN and Line-OUT current/voltage sensors 154 and 165, the MCU 102determines whether in any given phase there is an absence of measurablevoltage on both the Line-IN side and Line-OUT side of the FET powermodule 106. If there is (“YES” at decision 1006), the MCU 102 is able toconclude that one or more phases of the air gap disconnect unit 108has/have failed open. Accordingly, at step 1008 the MCU 102 reports tothe system overseer, over the comm/control bus 124, that one or morephases of the air gap disconnect unit 108 has/have failed open.

If the MCU 102 determines that for each of the three phases there is notan absence of measurable voltage on both the Line-IN and Line-OUT sidesof the FET power module 106 (“NO” at decision 1006), it is stillpossible that one or more of the Line-IN and Line-OUT current/voltagesensors 154 and 156 has/have failed or is/are likely failing. To furtherascertain and isolate any failed or likely failing Line-INcurrent/voltage sensors(s) 154 or failed or likely failing Line-OUTcurrent/voltage sensors(s) 156, steps 1010-1022 are performed.Specifically, at decision 1010 the MCU 102 determines from Line-IN andLine-OUT voltage measurements taken by the Line-IN and Line-OUTcurrent/voltage sensors 154 and 156 if there is a voltage discrepancybetween the Line-IN and Line-OUT voltages in any of the three phases. If“NO,” the MCU 102 is able to conclude that all of the Line-IN andLine-OUT voltage sensors 154 and 156 and the air gap disconnect unit 108are all working properly and the method 1000 ends. On the other hand, ifat decision 1010 the MCU 102 determines that there is a voltagediscrepancy between the Line-IN and Line-OUT voltages in any of thethree phases (“YES” at decision 1010), the MCU 102 is able to properlyconclude that one or more of the voltage sensors 154 and/or 156 has/havefailed or is/are likely failing. To determine whether the failed orfailing voltage sensors in any failed phase is a Line-IN current/voltagesensor 154 or is a Line-OUT voltage sensor 156, at step 1012 the MCU 102directs the sense and drive circuit 104 to switch OFF the power FETs 116in each phase that it determined in decision 1010 that one or more ofthe voltage sensors 154 and/or 156 has/have failed or is/are likelyfailing, and at decision 1014 the MCU 102 then determines whether ineach of these phases whether V(Line-OUT) goes low or stays high. IfV(Line-OUT) does not go low in any of the phases (“NO” at decision1014), the MCU 102 concludes that one or more of the Line-OUT voltagesensors 156 has/have failed or is/are likely failing and/or that one ormore of the power FETs 116 has/have failed and, acting on thatdetermination, in step 1016 prevents any further attempt to switch thepower FETs 116 back ON. Then at step 1018 the MCU 102 reports to thesystem overseer that one or more of the Line-OUT voltage sensors 156has/have failed or is/are likely failing and/or one or more of the powerFETs 116 has/have failed, and/or directs the SSCB's 100's electronicdisplay 113 to indicate that one or more of the Line-OUT voltage sensors156 has/have failed or is/are likely failing and/or that one or more ofthe power FETs 116 has/have failed. If, on the other hand, the MCU 102determines at decision 1014 that V(Line-OUT) goes low in all phasesafter the power FETs 116 are switched OFF at step 1012 (“YES” atdecision 1014), the MCU 102 concludes that one or more of the Line-INvoltage sensors 154 has/have failed or is/are likely failing. Acting onthat determination, at step 1020 the MCU 102 prevents any furtherattempt to switch the power FETs 116 back ON. Finally, at step 1022 theMCU 102 reports to the system overseer that one or more of the Line-INcurrent/voltage sensors 154 has/have failed or is/are likely failingand/or directs the SSCB's 100's electronic display 113 to indicate thatone or more of the Line-IN voltage sensors 154 has/have failed or is/arelikely failing.

The solenoid 118 in the air gap disconnect unit 108 requires asignificant amount of energy to drive the air gap disconnect unit 108open. To avoid undesirable dips in the DC voltage VDC produced by theAC/DC converter 126 (which, as explained above, serves as the DC powersupply for the DC electronics in the SSCB 100, including the MCU 102,CRM 103, and DC components on the sense and drive circuit 104), thelarge air gap disconnect capacitor 117 is used as an energy source totrigger the solenoid 118. (Note: In some embodiments of the inventionthe air gap disconnect capacitor 117 is also configured to serve as abackup DC power supply for a short duration, supplying a backup DCvoltage VDC(backup), in the event of loss of the DC voltage VDC producedby the AC/DC converter 126.) In one embodiment of the invention, the MCU102 is programmed to monitor and diagnose the vitality, efficacy, andoperability of the air gap disconnect capacitor 117, and generate acapacitor bypass signal that allows the AC/DC converter 126 to bypassthe air gap disconnect capacitor 117 upon determining that the air gapdisconnect capacitor 117 has failed or is failing. FIG. 11 is aflowchart illustrating this method 1100. The method 1100 is performed asthe air gap disconnect capacitor 117 charges, in particular, each timethe SSCB 100 boots up and every time it is recharging after having beendischarged due to firing the solenoid 118 in the air gap disconnect unit108. During first step 1102, as the air gap disconnect capacitor 117charges, the MCU 102 measures the charging voltage rate dV/dt(measured).At step 1104 the MCU 102 compares the measured charging ratedV/dt(measured) to an expected (predetermined) charging ratedV/dt(expected), and then at decision 1106 determines whetherdV/dt(measured) significantly exceeds or significantly falls below theexpected charging rate dV/dt(expected). If it does not (“NO” at decision1106), the MCU 102 is able to conclude that the air gap disconnectcapacitor 117 is operating properly and the method 1100 ends. On theother hand, if at decision 1106 the MCU 102 determines that the measuredcharging rate dV/dt(measured) is significantly greater than orsignificantly less than the expected charging rate dV/dt(expected)(“YES” at decision 1106), at step 1108 the MCU 102 concludes that theair gap disconnect capacitor 117 has failed or is likely failing and theMCU 102 generates and applies a cap bypass control signal that closes acap bypass switch 123 (see FIG. 1 ) to bypass the air gap disconnectcapacitor 117. In this bypass configuration the AC/DC power converter127 is used to trigger the air gap disconnect unit 108. Since this isnot a preferred configuration (it instead being preferred to use theenergy stored in the air gap disconnect capacitor 117 to trigger the airgap disconnect unit solenoid 118), at step 1110 the MCU 102 reports tothe system overseer that the air gap disconnect capacitor 117 has failedor is likely failing and needs to be replaced. Additionally (oralternatively), the MCU 102 can also direct the SSCB's 100's electronicdisplay 113 to indicate that the air gap disconnect capacitor 117 hasfailed or is likely failing.

Because the MCU 102 is the ‘brains’ of the SSCB 100, it is importantthat it not fail, but in the unlikely event that it does fail it ispreferable to have some way to trip the SSCB 102 without the requiredassistance of the MCU 102. To fulfill this goal, and as illustrated inFIG. 12 , in one embodiment of the invention the MCU 102 is programmedto produce a periodic ‘heartbeat’ (e.g., a 10 kHz square wave) at one ofits outputs. The heartbeat is fed to the input of an external ‘watchdog’1202, which produces an output signal that switches OFF the power FETs116 (or that is used to direct the sense and drive circuit 104 to switchthe power FETs 116 OFF). Additionally, the watchdog 1202 generates asolenoid trigger signal that triggers the air gap disconnect unit 108 todisengage (open air gap contact switches 114), similar to how the MCU102 generates a solenoid trigger signal to trigger the air gapdisconnect unit 108 to disengage when the MCU 102 is operating properly.Alternatively, the watchdog 1202 is configured to first attempt to resetthe MCU 102, before switching the power FETs 116 OFF and beforetriggering the air gap disconnect unit 108. The watchdog 1202 can beconstructed in various ways. In one embodiment of the invention itcomprises a counter and a flip-flop, which together monitor the MCU's102's heartbeat and generate disable signals to switch OFF the powerFETs 116 and disengage the air gap disconnect unit 108 when theheartbeat flatlines or becomes erratic or aperiodic.

In the exemplary embodiment of the SSCB 100 described herein the AC/DCconverter 126 (see FIG. 1 ) serves as the primary DC power supply forthe MCU 102, CRM 103, sense and drive circuit 104, and other DC-poweredelectronics within the SSCB 100. In one embodiment of the invention theAC input power to the AC/DC converter 126 is AC mains power supplied bythe AC mains. Consequently, it is important to monitor the presence ofthe AC mains power and the vitality and operability of the AC/DCconverter 126, to ensure the SSCB's 100's intended and proper operation,and to take appropriate measures in the event that the AC input power islost and/or the AC/DC power converter 126 fails. In one embodiment ofthe invention the presence of AC input power is continually monitoredand the vitality, efficacy, and operability of the AC/DC converter 126is also monitored. If the AC input power is determined to be not presentand/or the AC/DC converter 126 is determined to have failed ordetermined to be likely failing, the MCU 102 directs the sense and drivecircuit 104 to switch the power FETs 116 OFF (using energy stored in theair gap disconnect capacitor 117, if necessary). FIG. 13 is a flowchartfurther illustrating this method 1300. While monitoring the AC/DCconverter's 126's DC output voltage VDC in step 1302, at decision 1304the MCU 102 determines whether VDC is less than some predetermined lowDC threshold VDC(threshold), i.e., determines whetherVDC<VDC(threshold). If the MCU 102 determines that VDC has dropped belowthe threshold VDC(threshold) (“YES” at decision 1304) and that VDC hasremained below the threshold for longer than some predetermined durationof time and at decision 1306 the MCU 102 also determines that AC poweris present at the input of the AC/DC converter 126, the MCU 102 is ableto properly conclude that the AC/DC converter 126 has failed or islikely failing. Accordingly, at step 1312 the MCU 102: directs the senseand drive circuit 104 to switch the power FETs 116 OFF, generates asolenoid trigger signal that causes the air gap disconnect unit 108 todisengage and open air gap contact switches 114, and prevents anyfurther attempts to switch the power FETs 116 back ON and re-engage theair gap disconnect unit 108, until an electrician or engineer can bedispatched to replace the SSCB 100 or remove the failed or failing AC/DCconverter 126 and replace it with one that is functional. If at decision1306 the MCU 102 determines that AC power is not present at the input ofthe AC/DC converter 126, the MCU 102 cannot conclude that VDC hasdropped below the threshold VDC(threshold) due to a failed or failingAC/DC converter 126. So that the SSCB 100 can resume its normaloperation in the event that the AC power is restored, in step 1312 thecurrent state of the SSCB 100 may be stored in the flash memory portionof the SSCB's 100's CRM 103.

The air gap disconnect unit 108 is designed to galvanically isolate theload upon the SSCB 100 detecting a fault or overload of unacceptablylong duration. To prevent arcing across the air gap, it is preferable toswitch the power FETs 116 OFF before the air gap disconnect unit 108completes forming the air gap between the Line-IN and Line-OUT terminals110 and 112. To accomplish this safeguard, in one embodiment of theinvention the MCU 102 is programmed to continuously monitor thevitality, efficacy, and operability of the power FETs 116. Then, upondetermining that one or more of the power FETs 116 has failed (or is/arelikely failing), the MCU 102 is prohibited from generating the solenoidtrigger signal that triggers the air gap disconnect unit 108 to open theair gap contact switches 114. FIG. 14 is a flowchart illustrating thismethod 1400 in more detail. First, at step 1402 the power FETs 116 inthe FET power module are switched OFF. Then, at steps 1404 and 1406 thecurrent/voltage sensors 154 and 156 sense/measure the voltages for eachphase at both the Line-IN and Line-OUT sides of the FET power module106. Using the measured voltages, at step 1408 the MCU 102 thendetermines whether there is a voltage drop ΔV across the drain-sourceterminals of any one of the power FETs 116. Since the power FETs 116were switched OFF in step 1402, a voltage ΔV should be present. If avoltage ΔV is determined to be present (“YES” at decision 1410), the MCU102 is able to properly conclude that the power FETs 116 are in factswitched OFF and operating as intended, and the method 1400 ends.However, if the MCU 102 determines that a voltage drop ΔV does notappear across any one of the drain-source terminals of any one of thepower FETs 116 or only a very small voltage drop ΔV appears (“NO” atdecision 1410), the MCU 102 is able to conclude that one or more of thepower FETs 116 has failed closed (or has likely failed closed or isfailing closed). Acting on this determination at step 1412 the MCU 102then prevents the air gap disconnect unit 108 from disengaging (so nopossibility of arcing can occur between the Line-IN and Line-OUTterminals 110 and 112) and prevents any further attempt to switch thepower FETs 116 back ON. Finally, at step 1414 the MCU 102 reports to thesystem overseer that one or more of the power FETs 116 has failed (orhas likely failed or is failing) and/or directs the SSCB's 100'selectronic display to indicate that one or more of the power FETs 116has failed (or has likely failed or is failing).

Alternative to the method 1400 depicted in FIG. 14 , after determiningthat ΔV is equal to or slightly greater than zero at decision 1410, theMCU 102 is programmed to command the air gap disconnect unit 108 totrigger and form the air gap, but only if the air gap disconnect unit108 has not already exceeded some predetermined maximum number of airgap arc exposures. In this way, in the event that one or more of thepower FETS 116 has/have failed closed and so long as the maximum numberof air gap arc exposures has not been reached, an air gap can bebeneficially formed between the Line-IN and Line-OUT terminals 110 and112 to galvanically isolate the load. FIG. 15 is a flowchartillustrating this alternative method 1500. First, after determining thatΔV is not greater than zero at decision 1410, at decision 1502 the MCU102 determines whether the predetermined maximum number of air gap arcshas been reached. If not (“NO” at decision 1502), at step 1504 the MCU102 commands the air gap disconnect unit 108 to form an air gap betweenthe Line-IN and Line-OUT terminals 110 and 112, despite the fact thatone or more of the power FETs 116 may have failed closed and even thoughan arc might possibly occur between the Line-IN and Line-OUT terminals110 and 112. Then at step 1506 the MCU 102 reports the probable powerFET failure to the system overseer and/or directs the SSCB's 100'selectronic display 113 to indicate that one or more of the power FETs116 has/have failed or likely failed. On the other hand, if the MCU 102determines at decision 1502 that the predetermined maximum number of airgap arcs has been reached (“YES” at decision 1502), at step 1508 the MCU102 triggers the mechanical lock out mechanism 160 to ensure that theair gap disconnect unit 108 cannot be re-engaged.

In a preferred embodiment of the SSCB 100, positive temperaturecoefficient (PTC) thermistors 152 (see FIG. 1 ) are mounted in the FETpower module 106 as near as possible to each of the power FETs 116. Thethermistors 152 are configured to measure and report the real-timeoperating temperature of the power FETs 116, and the MCU 102 isconfigured to receive and respond to the temperature measurementsaccording to the method 1600 depicted in FIG. 16 . Specifically, inresponse to the sensed/measured power FET junction temperaturessensed/measured by the thermistors 152 and reported to the MCU 102 (step1602), at step 1604 the MCU 102 determines whether the sensed/measuredjunction temperature T_(J) of any one of the power FETs 116 is greaterthan a predetermined maximum permissible junction temperature T_(MAX),i.e., whether T_(J)>T_(MAX). If not (“NO” at decision 1604) the method1600 loops back to step 1602. However, if the MCU 102 determines thatthe sensed/measured junction temperature T_(J) of any one of the powerFETs 116 is greater than the predetermined maximum permissible junctiontemperature T_(MAX) (“YES” at decision 1604), the MCU 102 is able toconclude that a possible thermal runaway condition is developing in oneor more of the power FETs 116. To prevent damage to the power FET(s) 116and/or possibly other components in the SSCB 100, at step 1606 the MCU102 commands the air gap disconnect unit 108 to disengage to interruptcurrent flow through the power FETs 116, and at step 1608 the MCU 102reports to the system overseer that a thermal runaway condition haslikely occurred and/or directs the SSCB's 100's electronic display 113to indicate that a probable thermal runaway condition has likelyoccurred. Additionally (or alternatively), the MCU 102 may also directthe sense and drive circuit 104 to switch the power FETs 116 OFF.

In the exemplary embodiment of the SSCB 100 disclosed herein, Line-INand Line-OUT current and voltage (current/voltage) sensors (e.g., Halleffect sensors) 154 and 156 are configured near or at the inputs andoutputs of the FET power module 116. These measurements allow the MCU102 to determine whether any one of the power FETs 116 has failed closed(as described above). In one embodiment of the invention, the MCU 102 isprogrammed to further determine that one or more of the power FETs 116has failed (or is likely failing) by determining whether there is animbalance of Vdrop across one power FET compared to the others. Undernormal operating conditions, the RMS value of the line voltages shouldbe substantially the same, so at any given time the RMS value of Vdropacross all three power FETs 116 should also be substantially the same.Any significant imbalance in Vdrop among the three phases provides anindication that one or more of the power FETs 116 has failed or ispossibly failing. FIG. 17 is a flowchart highlighting a method 1700 theMCU 102 performs to detect such an imbalance and the steps it takes whenan imbalance in Vdrop is determined to be present. First, with all powerFETs 116 ON, at steps 1702 and 1704 the Line-IN and Line-OUTcurrent/voltage sensors sense/measure and report the Line-IN andLine-OUT voltages present at the input and output of the FET powermodule 106 for all three power FETs 116. Next, using the sensed/measurevoltages, at step 1706 the MCU 102 calculates Vdrop across each powerFET 116. At decision 1708 if the values of Vdrop are the same for allthree power FETs 116 (“NO” at decision 1708), the method 1700 returns tostep 1702. However, if the MCU 102 determines that there is asignificant imbalance of Vdrop in one power FET 116 compared to theothers (“YES” at decision 1708), the MCU 102 is able to conclude thatone or more of the power FETs 116 has failed or is likely failing.Accordingly, at step 1710 the MCU 102 directs the sense and drivecircuit 104 to switch the power FETs 116 OFF and commands the air gapdisconnect unit 108 to form an air gap between the Line-IN and Line-OUTterminals 110 and 112. Then, in step 1712 the MCU 102 reports to theoverseer that one or more of the power FETs 116 has failed is likelyfailing and/or directs the SSCB's 100's electronic display 113 toindicate that one more of the power FETs 116 has failed or is likelyfailing.

In the method 1700 just described in reference to FIG. 17 , the MCU 102is programmed so that it is able to determine that one or more of thepower FETs 116 has/have failed (or is/are likely failing) when itdetermines there is an imbalance of Vdrop among the three power FETs116. In a single-phase version of the SSCB 100, where just a singlepower FET is employed, the MCU 102 is programmed to calculate Vdrop(again from real-time Line-IN and Line-OUT voltage measurements butacross just a single power FET) and determine based on Vdrop whether thesingle power FET has failed. This method 1800 is depicted in theflowchart presented in FIG. 18 . First, at steps 1802 and 1804current/voltage sensors on both the Line-IN and Line-OUT side of thepower FET 116 are sensed/measured and reported to the MCU 102. At step1806 the MCU 102 then computes Vdrop across the single power FET 116,and at decision 1808, based on measured current flowing through thesingle power FET 116 and the power FET's known operating characteristics(e.g., as obtained from a characterization of the power FET duringdesign of the SSCB 100 and/or from operating characteristics provided bythe power FET manufacturer) determines whether Vdrop>Vds(expected). IfVdrop is close to Vds(expected), the method 1800 returns to step 1802.However, if the MCU 102 determines that Vdrop is significantly differentfrom Vds(expected) at decision 1708, the MCU 102 is able to properlyconclude that the single power FET 116 has failed or is likely failing.Accordingly, at step 1810 the MCU 102 directs the sense and drivecircuit 104 to switch the power FET 116 OFF and/or command the air gapdisconnect unit 108 to disengage and form an air gap between the Line-INand Line-OUT terminals. Finally, at step 1812 the MCU 102 alerts thesystem overseer that the power FET 116 has failed or is likely failing,so that an electrician or engineer can then be dispatched on site toreplace the SSCB 100 with a new SSCB or remove the defective powermodule 106 and replace it with a new power module 106 with a properlyfunctioning power FET 116. Additionally (or alternatively), the MCU 102can direct the SSCB's 100's electronic display to indicate that thepower FET 116 has failed or is likely failing.

In a preferred embodiment of the SSCB 100 the Line-OUT current/voltagesensors 156 include Hall effect sensors that produce Hall voltagesproportional to the magnetic fields formed around each of the Line-OUTconductors due to current flowing through the conductors. Because themagnetic field strengths are directly proportional to the magnitudes ofthe currents flowing through the Line-OUT conductors, the Hall voltagesare representative of the magnitudes of the currents flowing the SSCB100 when the power FETs 116 are switched ON and the air gap disconnectunit 108 is engaged (air gap contact switches 114 closed). Becauseaccurate Hall effect measurements are vital to the proper operation ofthe SSCB 100, in one embodiment of the invention the MCU 102 isprogrammed to monitor the performance and diagnose any current sensefailure. An exemplary method 1900 the SSCB 100 and MCU 102 perform inthis regard is presented in FIG. 19 . First, with all power FETs 116 ONand the air gap disconnect unit 108 engaged, at step 1902 the Halleffect sensors in the Line-OUT current/voltage sensors 156 sense/measurethe line currents flowing out of the Line-OUT terminals 112. Preferably,the Hall effect measurements (Hall voltages) are performed in real time,amplified if necessary, and reported to the MCU 102. At decision 1904the MCU 102 determines, based on the Hall voltages, whether the RMS ofany one of the three sensed/measured currents is significantly differentfrom the other two. If “YES”,” the MCU 102 is able to conclude that oneof the Hall effect sensors has failed or failing, so at step 1906directs the sense and drive circuit 104 to switch the power FETs 116 OFFand/or generates a solenoid trigger signal for the air gap disconnectunit 108 to trigger the air gap disconnect unit 108 to disengage andform an air gap between the Line-IN and Line-OUT terminals 110 and 112.Then, at step 1908 the MCU 102 reports the current sensor failure thesystem overseer and/or causes the SSCB's 100's electronic display 113 toindicate that one or more of the current sensors has failed or is likelyfailing. If at decision 1904 the MCU 102 determines that the threesensed/measured currents are balanced (RMS of all three sensed/measuredcurrents substantially the same) (“NO” at decision 1904), thatdetermination does not completely eliminate the possibility that one ormore of the Line-IN and/or Line-OUT current/voltage sensors 154 and 156has failed or may be possibly failing, so steps 1910 through 1914 anddecision 1916 are performed. Specifically, at steps 1910 and 1912 theLine-IN and Line-OUT voltages are measured for all three phases at theinputs and outputs of the FET power module 106. At step 1914 the MCU 102then calculates Vdrop=V(Line-IN)−V(Line-OUT) for all three phases.Finally, at decision 1916 if the MCU 102 determines that for any givenphase V(Line-IN) and V(Line-OUT) are both high and Vdrop is higher thanexpected while the current is low (“YES” at decision 1916), the MCU 102is able to conclude that one or more of the Line-IN and/or Line-OUTcurrent/voltage sensors 154 and 156 has failed or is likely failing andsteps 1906 and 1908 are performed. Otherwise (“NO” at decision 1916) themethod returns to step 1902.

The exemplary embodiment of the SSCB 100 described herein includes aRESET button 144 that a person can press to manually re-engage the airgap disconnect unit 108 and close the air gap between the Line-IN andLine-OUT terminals 110 and 112. After the RESET button 144 is pressedand the air gap contact switches 104 are closed, the person can thenpress the green ON button 107 to switch the power FETs 116 ON andthereby place the SSCB 100 in a fully ON state. If for any reasonpressing the RESET button 144 does not successfully re-engage the airgap disconnect unit 108 and a person then presses the ON button 107 toswitch the power FETs 116 ON, undesirable arcing can occur across one ormore of the air gap disconnect switches 114 when the mechanism isreleased. To avoid this problem, in one embodiment of the invention theMCU 102 and other cooperating components of the SSCB 100 are configuredto perform an air gap engagement verification method 2000, an exemplaryembodiment of which is depicted in FIG. 20 . This air gap engagementverification method 2000 operates based on the fact that whenever aperson presses the RESET button 144 to engage the air gap disconnectunit 108, the motion of the solenoid's plunger 128 causes a smallelectrical pulse to be generated in the solenoid's 118's coil as theplunger 128 is pushed out of the solenoid's housing to latch andre-engage the air gap disconnect unit 108. The presence and shape ofthis pulse is used by the MCU 102 in the air gap engagement verificationmethod 2000 to verify that the air gap disconnect unit 108 has in factproperly engaged. Specifically, at step 2002, upon the RESET button 144being pressed, the pulse that appears at the terminals of the solenoid's118's coil is amplified, filtered and directed to an input of the MCU102. Next, at decision 2004 the received pulse is compared by the MCU102 to an expected pulse. If the received pulse matches the expectedpulse (“YES” at decision 2004), the MCU 102 is able to conclude that theair gap disconnect unit 108 has properly engaged and, as a result,allows the ON button 107 to be pressed to switch the power FETS 116 ON.On the other hand, if the received pulse does not match the expectedpulse (or if no pulse is received by the MCU 102 when the RESET button144 is pressed) (“NO” at decision 2004), at step 2008 the MCU 102prevents any pressing of the ON button 107 to switch the power FETS 116ON. In this way, arcing is prevented from occurring across the air gapcontact switches 114 if the power FETs 116 were otherwise allowed to beswitched ON. Finally, at step 2010 the MCU 102 reports to the systemoverseer that the air gap disconnect unit 108 has likely failed openand/or directs the SSCB's 100's electronic display 113 to indicate thatthe air gap disconnect unit 108 has likely failed open.

In one embodiment of the invention each of the three phases of the SSCB100 includes a pair of back-to-back power FETs 116, rather than just asingle power FET, as illustrated in FIG. 21 . The back-to-back power FETconfiguration is desirable in some applications since among otherattributes it facilitates soft starting of inductive motor loads,similar to as explained in previously referred to and commonly assignedU.S. Pat. No. 10,541,530. During an instantaneous trip condition (e.g.,when one or more of the line currents exceeds some multiple of theSSCB's 100's rated current I_(RATED) (e.g., x6 I_(RATED)), the sense anddrive circuit 104 switches the power FETs 116 OFF. When the power FETS116 are switched OFF in this circumstance, a high-voltage ringing candevelop across the SSCB 100, specifically, between drain-to-drain of oneor more of the back-to-back power FETs 116. In the version of the FETpower module 106 depicted in FIG. 21 , surge protection devices (SPDs)2102, e.g., metal-oxide varistors (MOVs) ortransient-voltage-suppression (TVS) diodes, are connected across eachpair of back-to-back power FETs 116 in each phase. The SPDs 2102 turnON, during the instantaneous trip condition if the voltage developedacross them exceeds some predetermined threshold. When turned ON, theSPDs 2102 clamp the voltage across the drain-to-drain of eachback-to-back power FET configuration to prevent the high-voltageringing. However, the SPDs 2102 can only withstand so many high voltagesuppressions before they begin to degrade and eventually fail. In oneembodiment of the invention, the MCU 102 is configured to perform amethod 2200 that monitors the health of the SPDs 2102 and to lock downthe SSCB 100 if any one or more of the SPDs 2102 has failed orsufficiently degraded, so that an electrician or engineer can then bedispatched on site to service the SSCB 100 (e.g., by replacing the oneor more failed or degraded SPDs 2102) or replacing the SSCB 100 with anew SSCB 100 having new SPDs 2102. FIG. 22 is a flowchart depicting anexemplary embodiment of this SPD health monitoring method 2200. First,at step 2202 the MCU 102 constantly monitors the Line-IN and Line-OUTvoltages at the inputs and outputs of the SSCB 100. Next, upondetermining that an instantaneous trip condition is occurring atdecision 2204 (“YES” at decision 2204), the sense and drive circuit 104switches the power FETs 116 OFF at step 2206. Once the power FETs 116have been switched OFF, at decision 2208 the MCU 102 determines (basedon voltage measurements taken by the Line-IN and Line-OUTcurrent/voltage sensors 154 and 156) whether the voltage drop Vdropacross any pair of back-to-back power FETs 116, i.e., the voltage dropVdrop across any SPD 2102, is less than some predetermined minimumvoltage threshold (in one exemplary embodiment, less than 60% of theSPD's nominal voltage drop). (Note that since each SPD 2102 is connectedacross its associated back-to-back power FETs 116 drain-to-drain, Vdropacross each pair of back-to-back power FETs 116 is the same as thevoltage dropped across its associated SPD 2002.) In one embodiment ofthe invention Vdrop for each phase is the average voltage dropVdrop(avg) determined based on a series of instantaneous voltage dropmeasurements taken during the instantaneous trip condition event. Foreach phase, absolute instantaneous voltage drop measurements are takenon each execution cycle of the MCU 102, and the MCU 102 looks for aplateau by checking the instantaneous values to the previous values. Todetermine the average voltage drop Vdrop(avg) for each pair ofback-to-back power FETs 116, the absolute instantaneous voltage dropmeasurements in the plateau region are added to an accumulator for agiven current level. This loop also counts the time period of theplateau. Using the counted time period and accumulator value, theaverage voltage drop Vdrop(avg) is then computed for a defined current.This is repeated for all three phases, resulting in average clampvoltage levels and time periods of the three phase plateau regions. Dueto the nature of a three-phase system, two of the SPDs 2102 will beaffected in a similar fashion during an instantaneous trip conditionevent. As decision 2208 is being performed, the two affected phases areidentified by measuring current, and once identified the MCU 102determines whether Vdrop(avg) for either of the two identified phaseshas dropped below the predetermined minimum voltage threshold. If theMCU 102 determines at decision 2208 that all SPDs 2102 are still workingas intended (i.e., a “NO” at decision 2208), at step 2210 the MCU 102generates a solenoid trigger signal to cause the air gap disconnect unit108 to disengage and form an air gap between the Line-IN and Line-OUTterminals 110 and 112 and thereby respond to the instantaneous tripcondition that was previously detected at decision 2204. On the otherhand, if the MCU 102 determines at decision 2208 that Vdrop(avg) in anyof the three phases has fallen to a level less than the predeterminedminimum voltage threshold (i.e., a “YES” at decision 2208) the MCU 102is able to properly conclude that one or more of the SPDs 2102 has/havelikely failed or has/have sufficiently degraded (for having suppressedtoo many voltage surges) that they should be replaced. Accordingly, if a“YES” results at decision 2208, at step 2212 the MCU 102 generates asolenoid trigger signal to cause the air gap disconnect unit 108 todisengage and form an air gap between the Line-IN and Line-OUT terminals110 and 112. Finally, at step 2214, the MCU 102 alerts the systemoverseer that one or more of the SPDs 2102 in the SSCB 100 has/havelikely failed or has/have sufficiently degraded to a point that it/theyneed to be replaced and/or causes the SSCB's 100's electronic display113 to indicate that one or more of the SPDs 2102 has/have likely failedor has/have sufficiently degraded to a point that it/they need to bereplaced.

As explained in detail above, it is preferable to first turn the powerFETs 116 in the FET power module 106 OFF before forming the air gap, sothat arcing does not occur across the air gap. To facilitate thisoperation, in one embodiment of the invention the RELEASE button 122(see FIGS. 1-3 and 5 ) is equipped with a microswitch that opens andcloses depending on the physical displacement of the RELEASE button 122.(In a preferred embodiment of the SSCB 100, two tactile mechanicalswitches 115 (see FIG. 3 ) are used (two for redundancy).) Themicroswitch is in electrical communication with the MCU 102, allowingthe MCU 102 to monitor its status, i.e., open or closed. So long as theRELEASE button 122 is in its natural state (not pushed), the microswitchremains closed. However, upon a person pressing the RELEASE button 122and as the RELEASE button moves into the SSCB enclosure, the microswitchopens. When the MCU 102 detects this change in status of themicroswitch, it immediately responds by directing the sense and drivecircuit 104 to switch the power FETs 116 in the FET power module 106OFF. The power FETs 106 are switched OFF very quickly in this process,well before the air gap disconnect unit 108 is able to complete formingthe air gap. In this way, arcing across the air gap is prevented.

In one embodiment of the invention the computer program instructionsstored in the CRM 103 and retrieved and executed by the MCU 102 not onlyenable the MCU 102 to monitor the operational status of the one or moremicroswitches 115, they further include instructions that enable the MCU102 to: determine whether the one or more microswitches 115 has/havefailed, respond to any determined failure, for example, by locking downthe SSCB 100, and inform the system overseer of any failure. FIG. 23 isa flowchart illustrating a method 2300 the MCU 102 performs, accordingto this particular aspect of the invention. First, immediately followinga voltage collapse at the input of the SSCB 100 but not in DC power ofthe internal power supply (step 2302), at decision 2304 the MCU 102determines whether it received a change in status signal from the one ormore microswitches 115 indicative of a person pressing the RELEASEbutton 122. If “YES” at decision 2304, the MCU 102 is able to properlyconclude at step 2306 that the RELEASE button 122 was in fact pressed,and at step 2308 enters a fault state (power FETs 116 switched OFF andair gap disconnect unit 108 disengaged). If, on the other hand, the MCU102 does not receive a signal from the one more microswitches 115 (“NO”at decision 2304), the possibility still remains that a person did pressthe RELEASE button 122 but for some reason the MCU 102 did not receive asignal indicating that it had been pressed. To determine if the RELEASEbutton 122 was in fact pressed but that the one or more microswitches115 may have failed, decision 2310 is performed. Specifically, atdecision 2310 the MCU 102 queries as to whether the SSCB 100 initiatedthe command to trigger the air gap disconnect unit 108 (automatically bytriggering the solenoid 118) upon the Line-IN voltage sensors 154detecting the voltage collapse at the SSCB input. If it did (“YES” atdecision 2310), at step 2312 the MCU 102 concludes that the SSCB 100responded automatically to the fault (without a person pressing theRELEASE button 122) and at step 2314 commands the SSCB 100 to enter afault state (power FETs 116 switched OFF and air gap disconnect unit 108disengaged). On the other hand if the MCU 102 did not receive achange-in-status signal from the one more microswitches 115 (“NO” atdecision 2304) and the MCU 102 did not initiate the command to triggerthe air gap disconnect unit 108 (“NO” at decision 2310), the MCU 102concludes at step 2316 that a person did in fact press the RELEASEbutton 122 but for some reason the MCU 102 did not receive the signalfrom the one or more microswitches 115 upon the RELEASE button 122 beingpressed. In other words, at step 2216 the MCU 102 is able to properlyconclude that although a person did in fact press the RELEASE button 122the one or more microswitches 115 has/have likely failed. After makingthis determination, at step 2318 the MCU 102 directs the sense and drivecircuit 104 to switch OFF the power FETs 116, trigger the air gapdisconnect unit 108 to disengage, and indicate on the SSCB's 100'selectronic display 113 that the one or more microswitches 115 has/havelikely failed. Finally, at step 2320 the MCU 102 reports to the systemoverseer (via comm/control bus 124) that the one or more microswitches115 has/have likely failed and need to be replaced.

In the exemplary embodiments of the invention described above, the SSCB100 is designed so that it can perform the various diagnostic,maintenance, and self-protection methods without any human intervention.In some embodiments of the invention, the SSCB 100 is designed so thatmany of the various methods described above can also be (oralternatively be) performed on-demand, i.e., initiated by a user, forexample, by an electrician, engineer, or electrical utility. Tofacilitate this on-demand capability when the SSCB 100 is installed in adistribution panel 2402 (see FIG. 24 ), the SSCB 100, its MCU 102 inparticular, is programmed to communicate with an external user computer2406 via a wired (or wireless) head-end interface 2404 configured in thedistribution panel 2402. FIG. 25 is a drawing showing the principalcomponents of the user computer 2406, which may comprise a server,desktop computer, laptop computer, tablet computer, smartphone, or anyother type of computing device. As indicated in the drawing (FIG. 25 ),the user computer 2406 includes a microprocessor 2502; CRM 2504; ahuman-machine interface (HMI) 2506 through which the user can interactwith the user computer 2406; an electronic display 2508; and an optionalmass storage device 2510 (e.g., a magnetic hard drive or a solid-statedrive). The CRM 2504 is configured to store computer programinstructions that direct how the microprocessor 2502 operates. Thesecomputer program instructions include instructions and protocol thatprovide the microprocessor 2502 the ability to communicate with the MCUs102 in the various SSCBs 100 over the comm/control bus 124, via thehead-end interface 2404, instructions that allow the microprocessor 2502to individually address each one of the SSCBs 100, and instructions thatcontrol the times at which the microprocessor 2502 communicates with theMCUs 102 in the various SSCBs 100. In one embodiment of the invention,the user-interactive capability provided by the user computer 2406 ispresented in the form of a user-interactive graphical user interface(GUI) and computer program instructions stored in the CRM 2504 of theuser computer 2406 include instructions that direct the microprocessor2502 to generate and display one or more GUI windows or pages on theuser computer's electronic display 2508. Preferably, the electronicdisplay 2508 is equipped with touchscreen technology, which enables theuser of the user computer 2406 to interact with the GUI windows or pagesby touching the screen of the electronic display 2508 or by using astylus. Using simple or multi-touch gesture using one or more fingers,the user can scroll, zoom, input information, etc. and control what GUIwindows or pages and content are being displayed on the electronicdisplay 2508. The GUI and electronic display 2508 could alternatively(or additionally) be configured so that the user can interact with theGUI windows or pages and content using a mouse, touchpad, or othernon-touchscreen input device. To facilitate user-interactivity, the GUIwindows or pages preferably include icons and widgets, such as radiobuttons, sliders, spinners, drop-down lists, menus, combo and textboxes, scrollbars, etc. The computer program instructions that themicroprocessor 2502 executes to generate the GUI preferably comprises anapp hosted on the local gateway; however, it could alternativelycomprise a web service installed on a local sever or remotely (e.g., inthe cloud). Preferably, the GUI includes a calendar that allows the userto schedule regular occurrences of diagnostics and/or maintenance, anduser-interactive icons and/or widgets that allow the user to initiateon-demand diagnostics and/or maintenance. Whether scheduled oron-demand, the user computer's 2406's microprocessor 202 is preferablyprogrammed to report back a GUI window or page that displays the resultsof any diagnostics and/or maintenance run, any failures (or predictedfailures) of any of the various components of any given SSCB 100, andrecommended corrective actions. To further facilitate on-demanddiagnostics and/or maintenance, in one embodiment of the invention theSSCB 100 further includes an on-demand diagnostics button (e.g., labeled“test”), or, alternatively, the STANDBY button 111 is configured toserve as a dual purpose button which when pressed initiates theon-demand diagnostic and/or maintenance processes.

In the detailed description above, it was described that in oneembodiment of the invention the SSCB 100 includes a “secondary lockoutmechanism 160” that prevents the air gap disconnect unit 108 from beingengaged, upon the SSCB 100 determining that a hazardous condition ispresent or that one of the SSCB's principal components or criticalfunctions has/have failed or is likely failing. In that exemplaryembodiment of the SSCB 100, a “secondary solenoid” is employed toprevent the air gap disconnect unit 108 from being reengaged whenconditions warrant. In another embodiment of the invention, a similarmechanical lockout function is performed and achieved but withoutrequiring the addition of a secondary solenoid. FIGS. 26A-26B, 27A-27Band 28A-28B are drawings showing various views of a modified air gapdisconnect unit 108′ that is adapted to perform this alternativemechanical lockout function. FIGS. 26A and 26B are side (FIG. 26A) andtop plan (FIG. 26B) views of the modified air gap disconnect unit 108′when engaged (holster in ‘down’ and air gap contact switches 114closed). FIGS. 27A-27B are side (FIG. 27A) and top plan (FIG. 27B) viewsof the modified air gap disconnect unit 108′ when disengaged (holster‘up’ and air gap contact switches 114 open). And FIGS. 28A-28B are side(FIG. 28A) and top plan (FIG. 28B) views of the modified air gapdisconnect unit 108′ when mechanically locked out (holster locked in‘up’ position and air gap contact switches 114 open). The principalcomponents in the modified air gap disconnect unit 108′ that are addedto facilitate the mechanical lockout procedure include: a lockout rod162; a lockout rod spring 164; a lockout arm 166; and a linkage pin 168that mechanically couples the lockout arm 166 to the piston (i.e.,plunger) 128 of the solenoid 118.

The lockout rod 162 is inserted through and supported by first andsecond bushings (i.e., sleeves) 170 and 172 and moves laterally (rightand left in the drawings), depending on the rotational movement of thelockout arm 166, as will be explained in more detail below. The lockoutrod spring 164 is held between the first bushing 170 and a firstretaining clip 174 (e.g., a circlip such as an e-clip or c-clip), andthe lockout rod spring 164 is fully compressed when the modified air gapdisconnect unit 108′ is engaged, so that a first end 176 of the lockoutrod 162 presses against a ramp 178 (see FIG. 26A) on the holster 136. Ascan be best seen in FIG. 26B, the lockout arm 166 includes a primarycatch 182, which, when engaging the lockout arm 166, restricts thelateral movement of the lockout rod 162, and a secondary catch 184,which includes a small protrusion 186 that, depending on the operationalstate of the modified air gap disconnect unit 108′ (engaged (26A and26B), disengaged (27A-27B), or locked out (28A-28B)), engages acircumferential groove 188 formed in the lockout rod 162 near a secondend 190 of the lockout rod 162. The primary catch 182 includes twoappendages (or “fingers”) 194 that bestride the lockout rod 162, betweenthe second bushing/sleeve 172 and a second retaining clip 192, both whenthe modified air gap disconnect unit 108′ is engaged (FIGS. 26A-26B) anddisengaged (FIGS. 27A-27B), but which are decoupled from the lockout rod162 when the modified air gap disconnect unit 108′ is in the locked-outconfiguration (FIGS. 28A-28B). The secondary catch 184 engages thecircumferential groove 188 when the modified air gap disconnect unit108′ is engaged (FIGS. 27A and 27B). It also engages the circumferentialgroove 188 during brief moments when the modified air gap disconnectunit 108′ is transitioning from the engaged state (FIGS. 26A-26B;holster in ‘down’ and air gap contact switches 114 closed) to thedisengaged state (FIGS. 27A-27B; holster ‘up’ and air gap contactswitches 114 open). As will become more clear from a reading of thedescription that follows, by engaging the circumferential groove 188 thesecondary catch 184 serves as a failsafe that prevents the lockout rod162 from extending underneath the holster 136 and unwantedlymechanically locking out the SSCB 100 when conditions do not warrant.

FIG. 29 is a flowchart illustrating a method 2900 the SSCB 100 performswhen transitioning from the engaged state (FIGS. 26A-26B) to thedisengaged state (FIGS. 27A-27B), and, if the SSCB 100 deems itnecessary, from the disengaged state (FIGS. 27A-27B) to the mechanicallylocked-out state (FIGS. 28A-28B), when the SSCB 100 is modified toinclude the modified air gap disconnect unit 108′ depicted in FIGS.26A-26B, 27A-27B and 28A-28B, according to one embodiment of theinvention. Just prior to the START of the method 2900, it is assumedthat the SSCB 100 is operating normally (for example, is configured inan electrical circuit and protecting an attached load) with the modifiedair gap disconnect unit 108′ in the engaged state/configuration (asdepicted in FIGS. 26A and 26B). As explained above, when the modifiedair gap disconnect unit 108′ is in the engaged state the first end 176of the lockout rod 162 presses against the holster ramp 178 (see FIG.26A), due to the lockout rod spring 164 being fully compressed, betweenthe first retaining clip 174 and the second lockout rod bushing (sleeve)172. The primary and secondary catches 182 and 184 of the lockout arm166 also engage the lockout rod 162, holding it in place until themethod 2900 starts. At first step 2902 in the method 2900, either thesolenoid 118 is triggered by the SSCB's MCU 102 (for example, due to afault or other undesirable circuit anomaly or failure in the SSCB 100)or a person presses the RELEASE button 122 with the desire tomechanically disengage the modified air gap disconnect unit 108′. Inresponse to either action, the solenoid's plunger 128 is forcedtemporarily into the solenoid's 118's housing, causing the latch 130 torelease the holster 136 and the holster's normally-compressed spring(s)138 to lift the holster 136 and form an air gap between the Line-IN andLine-OUT terminals 110 and 112, as was described in detail above.Because the linkage pin 168 in the modified air gap disconnect unit 108′mechanically links the solenoid's plunger 128 to the lockout arm 166,the lockout arm 166 is caused to rotate (clockwise when viewed from theperspective of FIG. 26B) about pivot 180 as the solenoid's plunger 128is forced into the solenoid housing. This step is indicated in theflowchart as step 2904. As the lockout arm 166 rotates, the fingers 194of the primary catch 182 release from the lockout arm 162, as indicatedby step 2906, allowing the lockout rod spring 164 to partiallydecompress and the lockout rod 162 to move (to the left in thedrawings), as indicated by step 2908 in the flowchart. Note that theholster ramp 178 no longer presents a barrier to the lockout rod 162,once the holster 136 has been pushed up by the holster's 136'snormally-compressed spring(s) 138 (compare FIG. 26A to FIG. 27A). Alsonote that during the time the lockout arm 166 is rotating and thefingers 194 of the primary catch 182 are releasing from the lockout rod162, the protrusion 186 of the secondary catch 184 continues to engagethe circumferential groove 188, thereby preventing the lockout rod 162from extending further beneath the holster 136 and undesirablymechanically locking the modified air gap disconnect unit 108′. Afterthe solenoid's 118's plunger 128 is forced into the solenoid housing(whether electromagnetically by the MCU 102 triggering the solenoid 118or mechanically by a person pressing the RELEASE button 122), at step2910 the solenoid return spring 196 returns the plunger 128 to its homeposition. This causes the lockout arm 166 to rotate about pivot 180 inthe reverse direction to that described above (counterclockwise whenviewed from the perspective of FIG. 27B), as indicated by step 2912. Asthe solenoid's plunger 128 travels back to its home position and thelockout arm 166 rotates back, the protrusion 186 of the secondary catch184 eventually releases from the circumferential groove 188 formed inthe lockout rod 162 near the second end 190 of the lockout rod 162.However, during this time the fingers 194 of the primary catch 182reengage the lockout rod 162, as indicated by step 2914, to prevent thelockout rod 162 from moving (to the left in the drawings) anymore thanit already had in step 2908. Arriving at this moment in this exemplarymethod 2900, the modified air gap disconnect unit 108′ has completedtransitioning from the engaged state (FIGS. 26A-26B) to the disengagedstate (FIGS. 27A-27B).

There are various causes and reasons why the modified air gap disconnectunit 108′ transitions from the engaged state (FIGS. 26A-26B) to thedisengaged state (FIGS. 27A-27B). For example, a short circuit oroverload that has persisted for too long may have been detected by theSSCB 100 and caused the SSCB's MCU 102 to trigger the solenoid 118, or aperson may have simply pressed the RELEASE button 122 to force atransition to the disengaged state. Transitioning to the disengagedstate is an important safety feature since, once completed, an air gapis formed between the SSCB's 100's Line-IN and Line-OUT terminals 110and 112 and, consequently, the load and electrical wiring in the SSCB's100's load circuit and the source of the fault are galvanically isolatedfrom the rest of the electrical distribution system. In manycircumstances the cause or reason why the modified air gap disconnectunit 108′ disengaged can be easily corrected and, once corrected, theSSCB 100 can be reset by a person simply pushing the RESET button 144,to move the holster 136 down, close the air gap contact switches 114,and switch the power FETs 116 in the FET power module 106 back ON. Thereare other circumstances, however, where it is imprudent to allow theSSCB 100 to be reset. For example, the SSCB 100 may detect or determineby performing one of the self-diagnostic or self-maintenance tests ormeasurements described above that: a short circuit inside the SSCB 100or an overcurrent in the SSCB's 100's load circuit has not cleared aftera predetermined duration of time or is otherwise uncorrectable; one ormore of the power FETs 116 in the SSCB's FET power module 106 has/havefailed or is/are likely failing; one or more of the current and/orvoltage sensors 154 and 156 has/have failed or is/are likely failing;the AC/DC converter 126 has failed or is likely failing; the MCU 102 hasfailed or is likely failing; one or more of the SPDs has/have failed orworn beyond its/their predetermined lifetime(s); a critical component inthe SSCB 100 has overheated; a conduction path or connector has brokenor substantially degraded; etc. Detecting or measuring any one of thesecircumstances and conditions could give impetus or motivation to preventthe SSCB 100 from being reset. Accordingly, immediately after themodified air gap disconnect unit 108′ has completed transitioning to thedisengaged state (FIGS. 27A-27B), i.e., immediately after step 2914 inthe method 2900 (refer again to the flowchart in FIG. 29 ), at decision2916 the SSCB 100 determines whether the SSCB 100 should allow a personto press the RESET button 144 to reengage the modified air gapdisconnect unit 108′ or mechanically lock the modified air gapdisconnect unit 108′ so that it cannot be reengaged. If a mechanicallock-out is not required (“NO” at decision 2916), at step 2918 a personmay then press the RESET button 144 to reengage the modified air gapdisconnect unit 108′ and the method 2900 ends. Note that as the RESETbutton 144 is being pressed and the holster 136 is being forced down thefirst end 176 of the lockout rod 162 comes into contact with and travelsup the holster ramp 178. This causes the lockout rod 162 to movelaterally (to the right when viewed in the drawings) and allow the smallprotrusion 186 that juts from the secondary catch 184 to reengage thecircumferential groove 188 formed in the lockout rod 162. On the otherhand, if at decision 2916 the SSCB 100 determines that a transition tothe mechanically locked-out state is required (“YES” at decision 2916),the method 2900 continues at step 2920. As explained above, when themodified air gap disconnect unit 108′ is in the disengaged state (asdepicted in FIGS. 27A-27B), the lockout arm's 166's primary catch 182engages the lockout rod 162, preventing the lockout rod spring 164 frompushing the lockout rod 162 beneath the holster 136, but the protrusion186 of the secondary catch 184 no longer engages the circumferentialgroove 188 formed in the lockout rod 162. As will be clear from thedescription that follows, transitioning to the mechanically locked-outstate (FIGS. 28A-28B) results in the holster 136 being mechanicallylocked so that the holster 136 cannot be pressed down and so that theair gap contact switches 114 cannot be closed, even if the RESET button144 is pressed. Accordingly, at step 2920 the MCU 102 in the SSCB 100generates a solenoid trigger signal that triggers the solenoid 118 tofire (i.e., actuate)—a first time, if the modified air gap disconnectunit 108′ entered the disengaged state in response to a person pressingthe RELEASE button 122, or a second time, if the modified air gapdisconnect unit 108′ entered the disengaged state by the MCU 102previously triggering the solenoid 118 to actuate. Next, at step 2922,the lockout arm 166 rotates (clockwise when viewed from the perspectiveof FIG. 27B) about pivot 180, as the plunger 128 is pulled into thesolenoid housing. As the lockout arm 166 rotates, the fingers 194 of theprimary catch 182 release from the lockout arm 162, as indicated by step2924, and because the protrusion 186 jutting from the secondary catch184 no longer engages the circumferential groove 188, the lockout rodspring 164 is free to fully decompress (from its partially decompressedstate in the disengaged state) and push the lockout rod 162 underneaththe holster 136, as can be best seen in FIG. 28B. With the lockout rod162 now underneath the holster 136, the holster 136 is renderedphysically incapable of being pushed down, by a person pushing the RESETbutton 144, by the solenoid 118 triggering once again, or by any otherway. In other words, the modified air gap disconnect unit 108′ and SSCB100 are mechanically locked-out following completion of step 2924. Atoptional step 2926, the MCU 102 alerts a system overseer, for example,via the comm/control bus 124, that the SSCB 100 is mechanicallylocked-out, and/or directs the SSCB 100 to generate an indication orwarning on the SSCB's 100′ s electronic display 113 that the SSCB 100 ismechanically locked-out and in need of repair or replacement, and themethod 2900 then ends.

While various embodiments of the present invention have been described,they have been presented by way of example and not limitation. It willbe apparent to persons skilled in the relevant art that various changesin form and detail may be made to the exemplary embodiments withoutdeparting from the true spirit and scope of the invention. Accordingly,the scope of the invention should not be limited by the specifics of theexemplary embodiments but, instead, should be determined by the appendedclaims, including the full scope of equivalents to which such claims areentitled.

The invention claimed is:
 1. A solid-state circuit breaker (SSCB),comprising: a power semiconductor device; an air gap disconnect unitwith a closeable air gap connected in series with the powersemiconductor device, wherein at any given time the air gap disconnectunit is configured in one of three operational states: an engaged stateduring which the closeable air gap is closed, a disengaged state duringwhich the closeable air gap is open and a reset of the air gapdisconnect unit to the engaged state remains operational, a mechanicallylocked-out state during which the closeable air gap is open but a resetof the air gap disconnect unit to the engaged state is prevented, and acontroller configured to: monitor and diagnose the vitality andoperability of principal components and core functions of the SSCB whenthe air gap disconnect unit is configured in the engaged state;determine whether a principal component or core function of the SSCB hasfailed or is failing; and upon determining that a principal component orcore function of the SSCB has failed or is failing, cause the air gapdisconnect unit to transition from the engaged state to the disengagedstate and then from the disengaged state to the mechanically locked-outstate.
 2. The SSCB of claim 1, wherein the air gap disconnect unitcomprises: movable electrical contacts that close the closeable air gapor open the closeable air gap depending on the operational state of theair gap disconnect unit; a latch that maintains the movable electricalcontacts in a position that closes the closeable air gap when the airgap disconnect unit is configured in the engaged state; and a solenoidwhich when triggered by the controller a first time causes the latch torelease the movable electrical contacts so they no longer close thecloseable air gap and allow the air gap disconnect unit to transitionfrom the engaged state to the disengaged state, and that when triggeredby the controller a second time causes the air gap disconnect unit totransition from the disengaged state to the mechanically locked-outstate, wherein the controller is configured to trigger the air gapdisconnect unit the second time upon the controller determining that aprincipal component or core function of the SSCB has failed or isfailing.
 3. The SSCB of claim 2, further comprising a RESET button thatcan be pushed by a user to reset the air gap disconnect unit back to theengaged state after the controller has triggered the solenoid the firsttime but that cannot be pressed by the user to reset the air gapdisconnect unit to the engaged state after the controller has triggeredthe solenoid the second time.
 4. In a solid-state circuit breaker (SSCB)including a power semiconductor device connected in series with an airgap disconnect unit, a method of mechanically locking the SSCB, themethod comprising: configuring the power semiconductor device and theair gap disconnect so that electrical current is able to pass and flowto an attached load; monitoring the vitality and operability ofprincipal components and core functions of the SSCB as current flows tothe attached load; determining during monitoring whether a principalcomponent or core function of the SSCB has failed or is failing; andupon determining that a principal component or core function of the SSCBhas failed or is failing, mechanically locking the air gap disconnectunit and galvanically isolating the load, wherein mechanically lockingthe air gap disconnect unit and galvanically isolating the loadcomprises opening an air gap between line-side terminals and load-sideterminals of the SSCB and preventing electrical contacts of the air gapdisconnect unit from closing the air gap.
 5. The method of claim 4,further comprising displaying information relating to a determinedfailure of a principal component or core function of the SSCB on anelectronic display of the SSCB.
 6. The method of claim 4, wherein theair gap disconnect unit comprises: a holster with electrical contactsthat open or close an air gap between the line-side terminals andload-side terminals of the SSCB, depending on a position of the holster;a latch that, during normal operating conditions, holds the holster inan engaged position with the electrical contacts closing the air gap;and a solenoid, wherein mechanically locking the air gap disconnect unitand galvanically isolating the load includes actuating the solenoid afirst time to cause the latch to release the holster and allow the airgap disconnect unit to disengage and actuating the solenoid a secondtime to lock the holster and prevent the holster from being moved backto the engaged position.
 7. The method of claim 6, wherein mechanicallylocking the air gap disconnect unit and galvanically isolating the loadcomprises obstructing a path of movement of the holster in response toactuating the solenoid the second time.
 8. The method of claim 6,wherein the SSCB includes a RESET button that a user can press to resetthe holster back to the engaged position after the solenoid has actuatedthe first time but that cannot be pressed to reset the holster back tothe engaged position once the solenoid has actuated the second time. 9.The method of claim 4, further comprising reporting a determined failureof a principal component or core function of the SSCB to a computer thatis communicatively coupled to a controller in the SSCB.
 10. Asolid-state circuit breaker (SSCB), comprising: line-side terminals forreceiving electrical power from an electrical power source; load-sideterminals for receiving a load; a power semiconductor device disposedbetween the line-side terminals and load-side terminals controlled toremain ON during normal operating conditions and allow electricalcurrent to flow to the load; an air gap disconnect unit connected inseries with the power semiconductor device, between the line-sideterminals and load-side terminals; a controller configured to switch thepower semiconductor device off, and trigger the air gap disconnect unitto disengage and form an air gap between the line-side terminals andload-side terminals, upon detecting or determining that a principalcomponent or core function of the SSCB has failed or is failing; and alock-out mechanism that prevents the air gap disconnect unit from beingreengaged after the controller detects or determines that a principalcomponent or core function of the SSCB has failed or is failing, whereinthe air gap disconnect unit comprises: a holster with electricalcontacts that open or close the air gap depending on a position of theholster; a latch that, during normal operating conditions, holds theholster in an engaged position with the electrical contacts closing theair gap; and a solenoid with a plunger that is mechanically linked toboth the latch and the lock-out mechanism.
 11. The SSCB of claim 10,wherein the controller is configured to trigger the solenoid to actuatea first time and cause the latch to release the holster and the air gapdisconnect unit to disengage with the electrical contacts no longerclosing the air gap, upon detecting a fault or upon determining that aprincipal component or core function of the SSCB has failed or isfailing.
 12. The SSCB of claim 11, wherein the controller is furtherconfigured to trigger the solenoid to actuate a second time and causethe lock-out mechanism to lock the holster and prevent the holster frombeing moved back to the engaged position if the solenoid was triggeredthe first time due to a fault and the fault has not cleared after apredetermined duration of time or if the solenoid was triggered thefirst time due to the controller detecting or determining that aprincipal component or core function of the SSCB has failed or isfailing.
 13. The SSCB of claim 12, wherein the lock-out mechanismcomprises a lockout rod configured and controlled to move, in responseto the solenoid triggering the second time, to a position that obstructsa path of movement of the holster and prevents the electrical contactsof the holster from closing the air gap.
 14. The SSCB of claim 13,wherein the lock-out mechanism further comprises a lockout arm includinga primary catch and a secondary catch and: both the primary catch andthe secondary catch hold the lockout rod and prevent the lockout rodfrom obstructing the path of movement of the holster when the holster isin the engaged position, the primary catch but not the secondary catchholds the lockout rod after the air gap disconnect unit has disengagedin response to the solenoid actuating the first time, and neither theprimary catch nor the secondary catch holds the lockout rod after thesolenoid has actuated the second time, allowing the lockout rod to moveto the position that obstructs the path of movement of the holster andprevents the electrical contacts of the holster from closing the airgap.